Flight data recorders are monitoring and recording instruments, carried aboard an aircraft, which systematically monitor and store the instantaneous values of various aircraft parameters. Early recorders were analog electromechanical devices which periodically marked, in analog form, the value of a given airplane parameter on a moving wire or other permanent storage medium. The time of occurrence of the parameter was also suitably scribed into the medium opposite the mark for the sensed parameter.
Subsequently, digital flight data recorders have been developed which operate by converting each analog aircraft parameter into a corresponding digital signal, and storing the digital signals on a permanent storage medium such as magnetic tape.
The numerous mechanical parts employed in the analog and digital type electromechanical flight data recorders have rendered such units expensive to construct and bulky in design, requiring periodic maintenance of the mechanical parts. In addition, extraction of the stored data from these data recorders requires physical removal of the storage medium.
The development of solid state memory devices, such as electrically erasable read-only memory, has led to the design of all solid state flight data recorders. The solid state flight data recorders commonly employ a data acquisition system (DAS) which receives and processes the various aircraft input signals to be monitored and stored under the control of a central processing unit (CPU). The analog signals are converted to digital signals by the DAS and, under CPU control, are passed over a data bus to the solid state memory devices. Programming within the CPU controls the processing of input airplane signals to corresponding digital signals through the DAS and the subsequent transference of these digital signals to controlled locations in the solid state memory.
The signals representative of monitored aircraft parameters are typically either discrete level signals or analog signals. Discrete signals are typically switch positions and produce either a high or a low level output depending upon the status of the particular switch. A typical example in an aircraft is a squat switch, which indicates whether or not a load is being borne by the landing gear. The analog signals may be either straight DC signals, DC radiometric signals, synchro signals or AC radiometric signals. The DC signals are static in nature, and generally range between a minimum and maximum value for the parameter being monitored. DC radiometric signals are DC signals having a ratio representative of the value of the parameter being sensed. A typical DC ratiometric signal is that produced by a potentiometer having a DC voltage applied across its resistive element, with the wiper position indicative of the level of the sensed parameter. Thus, the ratio of the wiper voltage to the voltage across the potentiometer's resistive element represents the level of the parameter being monitored.
Synchros are commonly employed to indicate the angle of a parameter. A synchro sensor is normally excited by two reference AC signals and outputs three active AC signals. The relative phasing and amplitude between the active AC signals and the reference signals indicate the angle of the synchro and, thus, the angle of the sensed parameter.
A typical AC ratiometric aircraft signal is that produced by a linear variable differential transformer (LVDT). LVDTs are commonly employed to indicate the relative position of aircraft control surfaces. Here, the ratio of the LVDT output sense AC signal to a reference AC signal is indicative of both total deflection and direction of deflection of the control surface.
To accurately collect data from synchro and ratiometric-type sensors, therefore, the DAS should simultaneously collect and hold each signal associated with the multisignal-type sensor.
Further, it is desirable to minimize the overhead on the CPU in its accessing of data as collected by the DAS. In prior art flight data recorder designs, the CPU sends a request to the DAS asking for the value of a given aircraft parameter and this parameter is then selected, processed, and analog-to-digital converted by the DAS which then signals the CPU that the requested information is available. Since a large number of airplane parameters may be monitored by the flight data recorder, constant requests by the CPU on the DAS significantly increases CPU overhead.
Further, it is desirable to conform the flight data recorder such that it is capable of being conveniently modified to operate in any one of several different types of aircraft. To this end, the DAS is preferably configured such that its inputs may be assigned by the CPU to handle any analog or discrete input signal. Further, the levels of the various signals at the inputs of the DAS must often be scaled for proper processing within the DAS. For example, inasmuch as all input signals are analog-to-digital (A/D) converted, the DAS typically includes a conventional A/D converter. The accuracy of an A/D converter is a function of the signal level applied at the input to the converter. To minimize A/D converter errors, therefore, it is essential that each aircraft parameter sensor signal be scaled before being applied to the A/D converter. In order to assure a universal flight data recorder design, the scaling factors applied to each input signal should be under CPU control.